The present invention is directed to a functional polymer. More specifically, the present invention is directed to a functional polymer having unsaturated groups and produces a free radical upon exposure to actinic radiation.
Polymers are employed for numerous purposes in a wide variety of industries. Functional polymers that readily form films on a surface are highly desirable for use in the fields of lithography, optical data storage, decorative pigments, adhesives, cosmetics, paints, shellack, security applications or active and passive optical elements such as polarizers, optical retarders or color filters, electrophotographic imaging members, and the like.
In addition to polymers, acrylates, methacrylates and other unsaturated monomers are employed in coatings, adhesives, sealants, and elastomers, and may be cross-linked by ultraviolet light (UV) radiation or peroxide initiated free radical cure. Such monomers are typically low molecular weight compounds that may be volatile or readily absorbed through skin and may cause adverse health effects. Additionally, many such unsaturated monomers are unstable in compositions, thus they precipitate out of solution during storage reducing the shelf life of the composition. Further, many unsaturated monomers precipitate out of processing solution leaving an undesirable scum or residue, which may contaminate articles or manufacturing apparatus. Reducing the monomer content of compositions may overcome some of the foregoing problems, however, unsaturated monomers are often needed to provide the cross-linking component in a composition.
In addition to unsaturated monomers, photoinitiators employed in compositions sensitive to actinic radiation also create unwanted problems. Many photoinitiators are insoluble in aqueous or polar diluents because of their aromatic structures. As a result such photoinitiators precipitate out of solution to contribute to the undesirable scum and residue caused by unsaturated monomers. Such a problem is especially found in a number of photoresists. Also, such photoinitiators may be absorbed through the skin creating a hazard to workers.
Photoresists include at least a resin binder, a cross-linking monomer or oligomer and a photoinitiator. A wide variety of polymeric binders may be used in photoresists. Such polymeric binders may include, as polymerized components, one or more acid functional monomers such as acrylic acid or methacrylic acid. Photoresists may be employed in a number of industries. Such industries include, but are not limited to, the electronics industry such as the manufacture of printed wiring boards, photomasks, planographic printing plates and semiconductors, color filters for use in color liquid crystal display devices, and color image pick-up elements.
A photoresist may be either positive-acting or negative-acting. For negative-acting photoresists, coating layer portions that are exposed to activating radiation polymerize or cross-link in a reaction between a photoactive compound and polymerizable agents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For positive-acting photoresists, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
Photoresists also may be either liquid or dry film. Liquid photoresists are disposed, coated, or applied on a substrate and then cured. Dry film photoresists may be laminated to a substrate. One problem with many photoresists is that they are difficult to strip from electrolytically plated circuit boards using conventional alkaline aqueous stripping solutions, e.g. 3% sodium hydroxide solutions. If the photoresist is not completely stripped and removed, ragged metal circuit lines may result after etching and may cause short-circuiting of the board.
Organic-based (amine- or organic solvent-containing) alkaline stripping solutions may be used which produce smaller stripped particles to facilitate stripping. While such organic-based strippers remove photoresist better than inorganic-based strippers, they are expensive relative to inorganic-based strippers (e.g. sodium or potassium hydroxide) and have more waste treatment and environmental concerns associated with them. Solvent-strippable photoresists are much less desirable due to workplace regulations limiting or reducing solvent emissions.
Another problem associated with many photoresists is the build-up of organic scum and residue from uncured photoresist, as briefly mentioned above. Such organic scum and residue may deposit on various articles and apparatus during the manufacture of products made using photoresists such as printed wiring boards, developer solutions and developer apparatus. Much of the organic scum and residue is caused by unsaturated monomers and oligomers such as (meth)acrylate-based compounds and photoactive agents having numerous aromatic groups. Examples of such photoactive agents that may form part of the scum and residue include, but are not limited to, imidazole dimers, benzophenones, acetophenones, anthraquinones, naphthaquinones, and triazine-based compounds. Such contaminants are not readily water-soluble or water-dispersible after they form residues in solution or deposit on an article or apparatus. As dissolved photoresists build up in solution (developer loading) insoluble organic materials begin to form in the developing tank eventually forming scum or residue. Presence of anti-foam additives (added to developer solutions to minimize foaming) increases the tendency for residue and scum to form. As the level of scum builds chances increase for a redeposit of the scum and residue onto the developed circuit board. Such redeposited residues cause a retardation of etching solution (etching chemistries have difficulty penetrating organic residues) and cause plating inhibition. Where etch is retarded, circuit shorts form causing a defective circuit board. In addition to increasing potential for defective circuit boards, the residue and scum also make cleaning equipment difficult, thus increasing maintenance time and cost in circuit board manufacturing.
U.S. Pat. No. 5,945,489 and U.S. Pat. No. 6,025,410 both to Moy et al. (also see “Novel Resins That Cure Without Added Photoinitiator” by Sheridan et al. Chemistry III-New Chemistry, RadTech 2002, pages 462–474 (Technical Conference Proceedings)) disclose photosensitive oligomers that may be cross-linked without an added photoinitiator. The patents disclose that a Michael addition of acetoacetate donors to multifunctional acrylate receptor compounds yields polyesters with reactive pendent acrylate groups, which may be cross-linked in a subsequent curing reaction. The patents state that pendent methyl ketone substituents serve as an internal photoinitiator. Upon exposure to UV radiation, an acyl radical with the methyl substituent is believed to be formed which acts as a photoinitiator, thus photoinitiators are not added to compositions containing the oligomers. Such oligomers are liquid oligomers, which may be employed as decorative coatings on wood and metal substrates. Odor generated from unreacted photoinitiators and skin absorption of unreacted photoinitiators is avoided, thus compositions containing such oligomers may be employed in materials that include medical and food contact applications. However, such oligomers are not believed to be suitable for use in photoresists because they are not alkali developable, and are not photosensitive at wavelengths greater than 320 nm. Accordingly, the oligomers of Moy et al. are limited in their applications.
Accordingly, in view of the foregoing problems there is a need for photosensitive compositions having components that are more water-soluble or water-dispersible and eliminate or reduce contamination of articles.